Matter in Our Surroundings

1. Separation by Filtration

The process of removing insoluble solids from a liquid by using a filter paper is known as filtration.Filtration is used for separating insoluble substances from a liquid. The mixture of insoluble solid and the liquid is poured into a filter paper cone fixed in a funnel by using a glass rod as a guide (see Figure 61).The liquid passes through the filter paper and collects in the beaker kept below the funnel. The solid particles (being bigger in size) do not pass through the filter paper and remain behind on the filter paper.The solid substance left behind on the filter paper is called residue. The clear liquid obtained is called filtrate. In this way, a mixture of insoluble solid in a liquid is separated into the 'solid' and clear liquid A mixture of chalk and water is separated by filtration. When the mixture of chalk and water is poured on the filter paper fixed in a funnel, then clear water passes through the filter paper and collects as filtrate. The chalk particles remain behind on filter paper as residue. A mixture of sand and water can also be separated by filtration. When the mixture of sand and water is poured on a filter paper fixed in a funnel, then clear water passes through filter paper and collects as filtrate. The sand remains behind on the filter paper in the form of a residue. The 'chalk and water mixture' and 'sand and water mixture'which have been separated by filtration are heterogeneous mixtures of a solid in a liquid. This means that a heterogeneous mixture of a solid and a liquid can be separated by the process of filtration.Please note that filtration can separate only insoluble substances (or undissolved substances) present in a liquid.Filtration cannot remove any solid substances which are dissolved in a liquid.
In science experiments we use a filter paper as 'filter' for carrying out the process of filtration. But in everyday life, different kinds of 'filters' are used. For example, a wire-mesh, a piece of cotton, a piece of muslin cloth (fine cloth) or even a layer of sand, can be used as 'filters' for various purposes. The size of holes of the filter to be used depends on the size of the solid particles to be separated from the liquid.When we make tea, we add tea-leaves. Now, to separate used tea-leaves we pour the prepared tea over a tea-strainer (see Figure 64). The tea-strainer has a wire mesh in it which acts as a filter. The liquid tea passes through the small holes of a tea-strainer and collects in the cup below. The tea leaves (being bigger in size) do not pass through the wire mesh of tea-strainer. The tea-leaves remain behind on the tea-strainer.In this way, the used tea-leaves are separated from prepared tea by the method of filtration.
In many homes, drinking water is filtered by using special 'water filters' which remove bacteria present in it. In cities, gutter water is filtered through big metallic filters to separate solid materials present in it so as to avoid choking of underground drains (called sewers). The process of filtration plays an important role in the purification of water at water works (which supplies drinking water in a city). This is described below.